FIG. 2 is a sectional front view of a prior art solid electrolytic capacitor 1, and FIG. 3 is a perspective view of a capacitor element 2 (see, for example, Japanese Examined Patent Publication No. HEI4-19695(1992)).
The solid electrolytic capacitor 1 includes an aluminum case 3 having a top opening, the capacitor element 2 contained in the case 3, and a rubber packing 30 which seals the opening of the case 3. An upper edge portion of the case 3 is curved to fix the packing 30, and a plastic seat plate 31 is attached to the top of the case 3. Lead wires 21, 21 extend from the capacitor element 2 through the packing 30 and the seat plate 31, and then bent laterally.
As shown in FIG. 3, the capacitor element 2 includes a roll element 20 including an anode foil 4 of an aluminum foil coated with a dielectric oxide film and a cathode foil 5 of an aluminum foil, which are rolled together into a roll with a separator 6 of an insulative material such as paper interposed therebetween and fixed by a tape 26. The roll element includes a solid electrolyte such as a TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt impregnated therein, or an electrically conductive polymer layer provided therein. Lead tabs 25, 25 respectively extend from the anode foil 4 and the cathode foil 5, and the lead wires 21, 21 respectively extend from the lead tabs 25, 25.
Although the solid electrolytic capacitor 1 having such a construction is widely used, there is a market demand for a capacitor having a smaller size and a greater capacitance. To this end, a capacitor has been proposed whose cathode foil 5 is coated with a metal nitride film as described below (see, for example, Japanese Unexamined Patent Publication No. 2000-114108).
An explanation will be given to the principle of the capacitance increase of the capacitor by coating the cathode foil 5 with the metal nitride film. In general, the dielectric oxide film is not intentionally formed on the cathode foil 5, but formed by natural oxidation. Therefore, the capacitance C of the capacitor is equivalent to a capacitance obtained by connecting the capacitance Ca of the anode foil 4 and the capacitance Cc of the cathode foil 5 in series, and represented by the following equation:C=Ca×Cc/(Ca+Cc)=Ca×1/(Ca/Cc+1)
That is, if the cathode foil 5 has the capacitance Cc, the capacitance C of the capacitor is smaller than the capacitance Ca of the anode foil 4.
Where a film 50 of a metal nitride such as TiN is formed on the cathode foil 5 by sputtering or vapor deposition as shown in FIG. 4, however, molecules of the metal nitride supposedly intrude into the oxide film 51 to contact an aluminum base of the cathode foil 5. Therefore, the base and the metal nitride are electrically connected to each other, so that the cathode foil 5 has no capacitance. Thus, the capacitance of the capacitor can be increased without size increase of the capacitor.
A non-valve metal may be employed instead of the metal nitride for the formation of the film. A valve metal herein means a metal naturally coated with its oxide, and examples thereof include tantalum and niobium in addition to aluminum.
However, the aforesaid capacitor has the following drawbacks.
The capacitor of this type is required to have a lower ESR (equivalent series resistance) as well as a higher capacitance. To this end, the width of the anode foil 4 or the cathode foil 5 as measured vertically should be increased. However, the vertical widths of the anode foil 4 and the cathode foil 5 cannot be increased to greater than the width of the separator 6. Widening the separator 6 is contradictory to the size reduction of the capacitor.
If the anode foil 4 and the cathode foil 5 each have the same vertical width as the separator 6 or a vertical width slightly smaller than the width of the separator 6, the foils 4 and 5 are brought into contact with each other due to a rolling shift of the foils 4, 5, thereby increasing the possibility of an increase in leak current or a defective due to a short circuit. Therefore, the foils 4, 5 each have a width smaller than the width of the separator 6 by about 0.5 mm to about 1 mm.
In view of the fact that the cathode foil 5 coated with the metal nitride film has no capacitance, the inventor has conceived the idea of reducing the width of the cathode foil 5 coated with the metal nitride film for preventing the foils 4 and 5 from contacting each other.
It is therefore an object of the present invention to provide a capacitor having an increased capacitance, a reduced ESR and a smaller size while suppressing a defective due to contact between foils 4 and 5 of the capacitor.